Converting to and from Python lists in NumPy - Mini Project: Build & Apply

Converting to and from Python lists

📖 Scenario: Imagine you work in a small bakery. You have a list of daily sales numbers. You want to use NumPy to do some quick calculations, but first, you need to convert your Python list into a NumPy array. Later, you want to convert the NumPy array back to a Python list to share with your team.

🎯 Goal: Learn how to convert a Python list to a NumPy array and then convert it back to a Python list.

📋 What You'll Learn

Create a Python list with exact sales numbers

Import the NumPy library

Convert the Python list to a NumPy array

Convert the NumPy array back to a Python list

Print the final Python list

💡 Why This Matters

🌍 Real World

Data scientists often receive data in Python lists but use NumPy arrays for fast calculations. Knowing how to switch between these formats helps in data cleaning and sharing results.

💼 Career

This skill is essential for data analysts and scientists who work with numerical data and need to prepare it for analysis or visualization.

1

Create the sales list

Create a Python list called sales with these exact numbers: 15, 22, 13, 27, 19.

NumPy

# Create the sales list with the exact numbers
# Your code here

Hint

Use square brackets [] to create a list and separate numbers with commas.

2

Import NumPy and convert list to array

Import the NumPy library as np. Then create a NumPy array called sales_array by converting the sales list using np.array().

NumPy

sales = [15, 22, 13, 27, 19]
# Import numpy as np and convert sales to sales_array
# Your code here

Hint

Use import numpy as np to import NumPy. Use np.array() to convert the list.

3

Convert NumPy array back to Python list

Create a new variable called sales_list_again by converting the sales_array NumPy array back to a Python list using the tolist() method.

NumPy

import numpy as np

sales = [15, 22, 13, 27, 19]
sales_array = np.array(sales)
# Convert sales_array back to a Python list called sales_list_again
# Your code here

Hint

Use the tolist() method on the NumPy array to get a Python list.

4

Print the final Python list

Print the variable sales_list_again to display the Python list converted back from the NumPy array.

NumPy

import numpy as np

sales = [15, 22, 13, 27, 19]
sales_array = np.array(sales)
sales_list_again = sales_array.tolist()
# Print sales_list_again
# Your code here

Hint

Use print() to show the list on the screen.

Practice

1. What does the function np.array() do when given a Python list?

easy

A. It deletes the Python list.

B. It converts the Python list into a Python dictionary.

C. It converts the Python list into a NumPy array.

D. It converts the Python list into a string.

Solution

1. Step 1: Understand np.array() purpose

   The function np.array() is designed to take a Python list and create a NumPy array from it.

2. Step 2: Compare options with function behavior

   Only It converts the Python list into a NumPy array. correctly states that it converts a list to a NumPy array. Other options describe unrelated actions.

3. Final Answer:

   It converts the Python list into a NumPy array. -> Option C

4. Quick Check:

   np.array(list) = NumPy array [OK]

Hint: np.array() always makes a NumPy array from a list [OK]

Common Mistakes:

• Thinking np.array() converts to dict or string
• Confusing np.array() with list methods
• Assuming np.array() deletes data

2. Which of the following is the correct syntax to convert a NumPy array arr back to a Python list?

easy

A. arr.tolist()

B. arr.to_list()

C. tolist(arr)

D. list(arr.to)

Solution

1. Step 1: Recall the method to convert NumPy array to list

   The correct method is tolist() called on the array object.

2. Step 2: Check each option's syntax

   arr.tolist() uses arr.tolist(), which is correct. arr.to_list() has a wrong method name. Options C and D are invalid syntax.

3. Final Answer:

   arr.tolist() -> Option A

4. Quick Check:

   array.tolist() converts to list [OK]

Hint: Use arr.tolist() to get a Python list from NumPy array [OK]

Common Mistakes:

• Using arr.to_list() instead of arr.tolist()
• Trying to call tolist() as a standalone function
• Incorrect method chaining or syntax errors

3. What is the output of this code?

import numpy as np
lst = [1, 2, 3]
arr = np.array(lst)
print(arr.tolist())

medium

A. [[1], [2], [3]]

B. array([1, 2, 3])

C. TypeError

D. [1, 2, 3]

Solution

1. Step 1: Convert list to NumPy array

   The list [1, 2, 3] is converted to a NumPy array arr with values [1 2 3].

2. Step 2: Convert NumPy array back to list

   Calling arr.tolist() converts the array back to a Python list [1, 2, 3].

3. Final Answer:

   [1, 2, 3] -> Option D

4. Quick Check:

   arr.tolist() = original list [OK]

Hint: arr.tolist() returns the original list values [OK]

Common Mistakes:

• Expecting array output instead of list
• Confusing print output with array representation
• Thinking it returns nested lists

4. Identify the error in this code snippet:

import numpy as np
lst = [1, 2, 3]
arr = np.array(lst)
new_list = arr.to_list()

medium

A. arr.to_list() is not a valid method

B. np.array() cannot convert lists

C. lst should be a NumPy array already

D. Missing import statement

Solution

1. Step 1: Check method names on NumPy arrays

   The method to convert a NumPy array to a list is tolist(), not to_list().

2. Step 2: Identify the error in the code

   Using arr.to_list() causes an AttributeError because the method does not exist.

3. Final Answer:

   arr.to_list() is not a valid method -> Option A

4. Quick Check:

   Use arr.tolist(), not arr.to_list() [OK]

Hint: Use arr.tolist(), not arr.to_list() [OK]

Common Mistakes:

• Typing to_list() instead of tolist()
• Assuming np.array() fails on lists
• Forgetting to import numpy

5. You have a nested Python list nested = [[1, 2], [3, 4]]. Which code correctly converts it to a NumPy array and then back to a nested Python list?

hard

A. arr = np.array(nested)
new_list = list(arr)

B. arr = np.array(nested)
new_list = arr.tolist()

C. arr = list(nested)
new_list = arr.tolist()

D. arr = np.array(nested)
new_list = arr.to_list()

Solution

1. Step 1: Convert nested list to NumPy array

   Using np.array(nested) converts the nested list into a 2D NumPy array.

2. Step 2: Convert NumPy array back to nested list

   Calling arr.tolist() converts the 2D array back to the original nested Python list structure.

3. Final Answer:

   arr = np.array(nested) new_list = arr.tolist() -> Option B

4. Quick Check:

   np.array() + tolist() preserves nested lists [OK]

Hint: Use np.array() then tolist() for nested list conversion [OK]

Common Mistakes:

• Using list(arr) which flattens the array
• Calling arr.to_list() which is invalid
• Trying to convert list before np.array()